A conventional electromechanical system, as utilized for example in the aerospace industry, includes a synchronous machine that generates multi-phase AC power from a rotating shaft, e.g., coupled to a gas turbine engine, and DC excitation. In addition to operating in a generator mode, the synchronous machine operates as a starter (motor) to start the aircraft engine. Following a successful engine start the system initiates the generator mode. In such applications where an AC electrical machine is used as a starter, it is desirable to provide a high starting torque with a power conversion device (e.g., inverter) having a limited input voltage. It is often difficult to provide and maintain the required starting torque, particularly in the constant power region (corresponding the high speed range) of the starting operation, with a power conversion device having a limited input/output voltage. This limitation in voltage is typically due to a fixed DC link available as input to the power conversion device. This limitation in existing systems results in non-optimal starter performance due to insufficient torque or due to the requirement for increased voltage (i.e., exceeding a maximum level).
Conventionally, motor controllers for applications requiring a controlled torque use discrete sensors, such as resolvers, to determine rotor position in a rotating machine. In addition, sensorless techniques have been developed to derive rotor position for motor drive control. A conventional motor drive control system is shown in FIG. 1. The primary components of the system include a power source 50, a controller (drive) 20, a motor generator 30, and a speed/position sensor 40. The controller 20 includes inverter controls 26 that receive signals from the position sensor 40 (e.g., speed/rotor position) and the motor generator 30 (e.g., current, voltage). These signals are used to control the main inverter 22 and exciter inverter 24, thereby providing a conventional closed loop system to regulate current supplied to the main inverter 22 as a function of the speed of the motor generator 30, as will be appreciated by those skilled in the art. Typically, during start, output current of the exciter inverter 24 is controlled to achieve field weakening based on rotor speed.
In generator mode, DC excitation of an exciter field winding and rotation of the generator shaft by the engine causes the generation of a polyphase voltage that is rectified and coupled to a main rotor field winding, which causes a rotating magnetic field in a main stator coil to produce output power with regulated voltage at a point of regulation (POR) for delivery to an AC bus. The DC current flowing through the exciter field winding may be varied in amplitude to achieve the desired magnitude of AC voltage on the AC bus.
When the motor/generator 30 is used to start the engine, power from the power source 50 is coupled to the synchronous motor/generator via the main inverter 22, which supplies controlled AC power to the main stator windings of the machine, such that sufficient torque is produced by the motor/generator 30. This torque is produced by the interaction between the flux in the main rotor winding and the current (flux) established in the main stator coil. The frequency of the controlled AC power from the main inverter 22 is increased from 0 Hz (0 RPM) to a predetermined frequency corresponding to the angular speed of the motor/generator 30 at the end of start. The phase of the current for the supplied AC power input is controlled as a function of rotor position/speed to develop the desired torque for motor/generator 30.
As rotor speed increases, back electromotive force (emf) generated in the motor proportionally increases, and opposes the supplied voltage, thereby requiring increased supply voltage to create sufficient current, which produces torque for engine start. During the high speed range of the start mode, the supply voltage at the input terminals of the motor/generator 30 needed to achieve sufficient torque may result in a supply voltage amplitude that exceeds a maximum acceptable levels, resulting in the requirement for a high supplied DC link voltage. This DC link voltage is limited by practical considerations, such as the maximum voltage of the power supply, the component rating, etc.